1. Field of the Invention
The present invention relates to a projection screen for rear-projection television sets. In particular, the present invention relates to a projection screen, the configuration of which includes a horizontal refracting linear Fresnel lens sheet and a vertical refracting linear Fresnel lens sheet, wherein a ridge line of said horizontal refracting linear Fresnel lens sheet extends in said vertical direction and a ridge line of said vertical refracting linear Fresnel lens sheet extends in said horizontal direction.
2. Description of the Prior Art
A projection screen for rear-projection TV sets comprises a Fresnel lens unit for converging a projection beam to the watching side and a lenticular unit for dispersing the projection beam and forming a picture. The Fresnel lens unit and the lenticular unit can be formed into one or two sheets which are referred to as a single-screen type and a double-screen type respectively. Most of the home TV sets are of the latter type.
A circular Fresnel lens sheet comprises circular unit lenses which are laid out on a plurality of planes in such a way that their ridge lines form concentric circles. Having such a configuration, the circular Fresnel lens sheet produces an effect of converging the projection beam to the watching side. The lenticular lens sheet, on the other hand, comprises linear unit lenses which are laid out on a plurality of planes in such a way that their ridge lines are parallel to each other. Having such a configuration, the lenticular lens sheet produces an effect of dispersing the projection beam and forming a picture.
The projection screen described above, be it of the single-screen type or of the double-screen type, has its Fresnel lenses formed into a concentric shape. A metal pattern used for manufacturing concentric Fresnel lens sheets is trimmed into a rotary disc.
A circular Fresnel lens is manufactured by using a plane forming pattern, the metal pattern trimmed into a rotary disc cited above. The circular Fresnel lens is fabricated by adopting methods such as a `press technique` a `cast technique` and a `UV` technique. In the press technique, a transparent resin plane is pressed against a heated metal pattern. In the cast technique, transparent resin undergoes thermal polymerization inside a metal-pattern cell. In the UV technique, a metal pattern is coated with ultraviolet-ray hardened resin and a transparent resin plane is then placed as a cover on the ultraviolet-ray hardened resin. Finally, an ultraviolet ray is applied thereto for hardening the resin.
In either method, however, circular Fresnel lens sheets must be produced one by one in order to create a circular form. As a result, the producibility is poor and it is difficult to reduce the production cost in comparison to the lenticular lens sheet to which continuous production by means of the press technique can be applied.
In addition, the optical dispersion characteristic of a lenticular lens sheet used in the projection screen is normally wide in the horizontal direction but narrow in the vertical direction. It is thus desirable to use the optical system of the screen as a converging system in the vertical direction but for producing parallelly radiated rays in the horizontal direction. When a circular Fresnel lens sheet is used, however, the lenses are created into a concentric form. Accordingly, it is impossible to set optical systems in the vertical and horizontal directions independently of each other. As a result, the optical system is generally used as a converging system also in the horizontal direction by adjusting it to the vertical direction which has a small diffusion angle. Furthermore, as disclosed in Japanese Patent Laid-open Nos. S56-147140 and S61-75677, a circular Fresnel lens sheet in use may be a lens sheet in which the focal distance is changed as the lens is located away from the center toward the peripheral. In this case, none the less, the focus distance varies within the range of the converging system.
As disclosed in Japanese Patent Laid-open Nos. S56-74429 and S56-74430, on the other hand, the use of two overlapping linear Fresnel lens sheets with their lines perpendicular to each other is proposed as a substitute for the concentric circular Fresnel lenses wherein each linear Fresnel lens sheet is formed with its Fresnel lenses overlapped in parallel in one direction.
In the case of such linear Fresnel lenses, the metal pattern for creating a linear Fresnel lens can be formed by directly trimming the metal pattern into a roll.
When Fresnel lenses are employed, a Fresnel lens can be created by using a roll-shaped forming mold as described above. Accordingly, Fresnel lenses can be produced continuously by creating Fresnel lenses through extrusion of thermoplastic resin or the use of resin hardened by an ultraviolet ray. As a result, it is possible to enhance the production efficiency and to reduce the production cost.
When linear Fresnel lenses are used in a configuration wherein they cross each other, however, the number of lens sheets is incremented by one in comparison to the case of circular Fresnel lenses. The increased number of linear Fresnel lens sheets reduces the gain. In addition, in the case of circular Fresnel lenses, the vertical/horizontal outgoing radiation angles on the same horizontal/vertical coordinates are equal to each other. In the case of such linear Fresnel lenses, on the other hand, these angles are not equal to each other, giving rise to a problem of deteriorated picture qualities.
In addition, linear Fresnel lenses crossing each other can be used in the vertical and horizontal directions or by letting the lenses cross each other in a slanting direction at a lens angle of 45 degrees. In the case of the 45 degree inclination, a problem of a high material cost is encountered even if the linear Fresnel lens is formed by the press technique.
On the other hand, a lenticular lens is created for diverging light in a direction perpendicular to the surface of either of the lens sheets which constitute a projection screen as disclosed in Japanese Patent Laid-open Nos. S58-93043 and S60-263932. By bearing part of the function of diverging the light in the perpendicular direction in this way, the amount of a light-diffusing material to be mixed in the lenticular lens sheet can be reduced. As a result, the loss of the projection beam can also be decreased as well.
In the configuration described, however, the vertical/horizontal outgoing radiation angles on the same horizontal/vertical coordinates are not made equal to each other. Accordingly, luminance as well as chrominance variations are prone to appear. As a result, the proposed configuration does not solve the problem that the picture quality deteriorates.
In a projection screen, part of the projected beam is generally reflected by the surface of the lens, becoming a stray beam traveling inside the sheet. The stray beam gives rise to picture interference such as overlapping structures and a rainbow.
In the case of an optical system employing linear Fresnel lenses as described above, in particular, the stray beam forms a white band which expands in the horizontal direction. This white band is observed more strikingly than a rainbow of a circular Fresnel lens, giving rise to a problem of undesirable picture observation.